CHATTANOOGA, Tenn. (AP) — A single "pop" cut through the quiet morning. Those who heard it had a moment to ponder the noise.

On this ordinary Thursday, some thought a car had backfired, or maybe a tire had blown. Sgt. 1st Class Robert Dodge looked up from his computer in an Army recruiting office in a strip mall, more curious than alarmed.

Then a young man in a rented convertible re-aimed his rifle and unleashed a frenzy of bullets. These were the opening shots in a single-handed rampage against the military that seized this city for hours and reignited American fears about radicalization and homegrown terror. The shooter's motive remains a mystery.

Glass shattered, televisions exploded, bullets whizzed past the heads of servicemen at their desks and rooted in the walls behind them. In nearby restaurants and hair salons and shops, people dived for cover or stood paralyzed by fear.

Inside the five side-by-side recruiting offices, one for each branch of the military and the National Guard, no one panicked.

"They were being soldiers," said Keith Wheatley, the property manager, a Marine himself, who arrived moments after the attack. "That's part of their job description. They know that any given time they could take fire, that's what they do. They weren't crying or upset. They were just trying to figure out what to do next."

Dodge had taken command of the recruiting office near Chattanooga Metropolitan Airport barely a month before. But after four tours in Iraq, he knew the drill. As the bullets rocketed by, he hustled the others in his office to a secure closet.

They emerged when the firing subsided, amazed to find just one person shot in the leg, unaware that the worst was far from over. They would soon learn from arriving police officers that the man in the silver Mustang had made his way across town to a Navy and Marine Reserve center and crashed the gate.

Back at the recruiting center, they waited for word outside their shattered storefront, emblazoned with the seal of the U.S. Marine Corps: a bald eagle atop the Earth, clutching a scroll in its beak that reads "Semper Fidelis." Always faithful.

Now seven jagged holes scarred the globe at its feet.

___

Seven miles from the strip mall, Lance Cpl. Skip Wells swapped texts with his girlfriend, Caroline Dove, 400 miles away at her home in Savannah, Georgia. They had not seen each other for months, and she was planning a visit for the following week.

"Can't wait anymore," he typed Thursday morning from the Reserve center, tucked between an industrial park and a leafy riverside park.

"Yes you can honey," she responded.

Around the same time, hordes of Chattanooga police heading for the recruiting offices heard the Mustang had been spotted, and changed course toward the Reserve center.

The car pulled off the highway, snaked around two concrete barriers meant to slow approaching vehicles and punched through the green chain-link gate into the parking lot.

The driver got out.

Officers who had tailed him there "immediately and aggressively" engaged him, Police Chief Fred Fletcher said.

In Savannah, a new message from Wells appeared on Dove's screen: "ACTIVE SHOOTER."

She thought he was joking.

___

The gunman, Muhammad Youssef Abdulazeez, lived with his parents in an all-American suburb, with big houses and tidy lawns. Classmates described the Kuwait-born 24-year-old as an affable young man who made the Red Bank High School wrestling team and once offered a stranded neighbor a ride home in the snow.

Investigators are trying to piece together where along the way he went wrong.

He struggled for years with depression, his family said in a statement Saturday.

"There are no words to describe our shock, horror and grief," they wrote. "The person who committed this horrible crime was not the son we knew and loved."

Court records point to a volatile family life. His mother filed for divorce in 2009 and accused her husband of sexually assaulting her and abusing their children. She later agreed to reconcile.

Abdulazeez, tall and athletic, with an engineering degree from the University of Tennessee at Chattanooga, worked for a short time at a nuclear power plant in Ohio. Then a drug test tripped him up, a federal official briefed on the case told The Associated Press, and the company let him go.

He visited relatives in Jordan for several months last year, and when he came back, he resumed attending services at a mosque. He kicked soccer balls on his lawn and kept up with his friends, neighbors said. Sometimes, he shot pellet guns off the back deck, aiming at a red target hanging from a tree in the woods.

Friends who saw him in recent weeks told the AP they noticed nothing alarming

On a Monday morning in April, around 2 a.m., a Chattanooga police officer pulled over a 14-year-old gray Toyota Camry. The driver, Abdulazeez, had been swerving, speeding and stopping at green lights, according to the police report. His eyes were droopy and watering; he slurred his speech and mumbled. He smelled of alcohol and marijuana and had white powder around his nostrils.

Abdulazeez told the officer that he had crushed up caffeine pills and snorted them. His court date was set for July 30.

That night, the police snapped a mug shot of the man with a crooked smile and a bushy beard.

___

Marilyn Hutcheson was working at a glass company across the street from the Reserve facility when, around 11 a.m., she heard a fusillade of gunshots.

"I couldn't even begin to tell you how many," she said. "It was rapid fire, like pow-pow-pow-pow, so quickly. The next thing I knew there were police cars coming from every direction."

A gunbattle erupted across the street.

Abdulazeez carried at least three guns, the FBI said. He didn't wear body armor but had on a vest designed to carry extra ammunition.

Investigators described him as a "moving target" and said he fired round after round at police.

Chattanooga police officers swarmed the scene, Fletcher said, each armed with a .45-caliber pistol and an AR-15 rifle. Some police brass rushed from headquarters; officers at home off-duty threw on their uniforms and ran.

One of Abdulazeez's bullets tore into Officer Dennis Pedigo's ankle. His fellow officers dragged the fallen policeman to safety.

It felt as if the firefight raged for 20 minutes, Hutcheson said. Business owners along the industrial corridor threw their doors closed and locked them. They huddled with employees and customers and people passing by who fled for cover. They watched the news and peered out as more police cars screamed down the street.

They couldn't see much from the windows. The Reserve center is set back from the street on the Tennessee River, in a valley and behind a row of trees. Neighbors could only hear chaos.

"We're apprehensive," Hutcheson said into the phone as the store remained locked down. "Not knowing what transpired, if it was a grievance or terroristic, we just don't know."

The area inside the fences is about the size of two football fields, said Lance Cpl. Austin Handle, who transferred from the facility last month. The building sits in the middle, surrounded by a parking lot, with a separate gated lot attached in the back where military vehicles are stored. It's a small complex that draws little attention — the last place Handle imagined ever seeing on the news.

Images showed officers with their weapons drawn crouched behind police cars and running from one car to the next. The area is mostly surrounded by woods, and the officers appeared at times as though they weren't sure where the gunfire was coming from. Along a jogging path that runs near the Navy-Marine Reserve center, a sniper lay on the ground with his rifle, peering through a scope. An officer knelt beside him, also ready to fire.

On the front line, officers advanced as Abdulazeez rained bullets. They returned fire until he was dead.

"It is apparent by looking at the crime scene ... that these officers were under a tremendous amount of gunfire from this individual," FBI agent Ed Reinhold said, "and yet they continued to move forward against this target and engage him and eliminated that threat, saving numerous lives throughout this community."

By the time it was over, four decorated Marines and a sailor — three of whom survived combat missions — and their attacker lay dead or dying.

Staff Sgt. David Wyatt, from Burke County, North Carolina, called Marines his brothers, friends said. The former Boy Scout enlisted in 2004 and was deployed three times, twice to Iraq. Handle described him as a "man's man," always quick with a joke.

Navy Petty Officer 2nd Class Randall Smith, a father of three, was a reservist on active duty in Chattanooga. He died at the hospital two days later.

Sgt. Carson Holmquist enlisted in 2009, months after graduating from high school in tiny Polk County, Wisconsin. He served two tours of duty.

Gunnery Sgt. Thomas Sullivan, of Hampden, Massachusetts, had served for 18 years, including two tours in Iraq. He was a father figure to the younger men, Handle said, tough but kind. He earned two Purple Hearts. His death could earn him a third.Lance Cpl. Squire Wells, nicknamed Skip, came from a suburban Atlanta military family: His grandfather was in the Air Force, and his grandmother and mother were in the Navy. He loved flag football and American history and hoped to become a drill sergeant. The other Marines made fun of him, Handle said, because he was always so motivated, even during the most excruciating drills.

Dove, Wells' girlfriend, whom he had been texting in the moments before the murders, is on the way to becoming a Marine herself, having signed up just months ago.

"I love you," she texted desperately after news of the attack reached her.

Hours passed. She called and called.

"Hon, I need you to answer me please," she wrote.

Silence.

___

Galofaro reported from Louisville, Kentucky. Bynum reported from Savannah, Georgia. Also contributing were Associated Press writers Michael Biesecker, Kathleen Foody and Jay Reeves in Chattanooga; Eric Tucker, Ted Bridis and Lolita C. Baldor in Washington; Travis Loller and Kristin M. Hall in Nashville; Adrian Sainz in Memphis, Tennessee; Rebecca Reynolds Yonker in Louisville; and Areej Hazboun in Jerusalem.